Field of the Disclosure
The invention relates to passive mode-locked ring fiber laser based on a non-linear polarization rotation (NPR) architecture. In particular, the invention relates to the mode-locked ring fiber laser provided with a control scheme which is operative to monitor and control the optical power and spectrum of ultrashort optical pulses in real time so as to maintain the ring fiber laser in a stable mode locked regime.
Discussion of the Known Art
Solid state femtosecond lasers are successfully used in a variety of industrial and research applications. Among various configurations of solid state femtosecond lasers, fiber lasers are more compact, simple, reliable and efficient than lasers based on bulk components.
Particularly important to the scope of this disclosure is the NPR-based mode lock mechanism associated with a fiber oscillator which has a ring resonant cavity. Well known to one of ordinary skill, the fiber ring laser of this type generally includes a gain medium, i.e., a length of fiber doped with rare earth elements, polarization sensitive isolator, and two polarization controllers flanking the isolator. All of these components are optically interconnected with one another to define a ring resonant cavity which has positive or negative or all-normal dispersion segments.
In the passively mode-locked ring laser based on the NPR, the polarization controller is operable to optimize the polarization such that the peak of the pulse travels through the polarizing isolator. As a consequence of NPR, the center of the pulse acquires a different polarization than its wings. Therefore, the isolator shortens the pulses by acting, in conjunction with this rotation mechanism, as an artificial fast saturable absorber.
As known to one of ordinary skill in the laser art, a ring fiber oscillator based on the NPR has the following advantages:
Relatively simple structure;
Various operational regimes depending on total cavity dispersion, resonator length and fiber type;
Generation of ultrashort broadband pulses of about 100 fs and shorter at 1.06, 1.55 and 1.9 nm central wavelengths which correspond to respective ytterbium (Yb), erbium (Er) and thulium (Tm) emission peaks; and
Self-start.
However, this laser configuration has a serious drawback—sensitivity to external stresses and, as a consequence, unstable pulse generation. As known to one of ordinary skill in the art, this instability is caused by external thermal and mechanical stresses leading to birefringent fluctuation process which may change the phase relation between orthogonally polarized field components. The disturbed phase relationship tends to increase optical losses and eventual loss of the mode locked regime which in general is characterized by a number of pulses, bandwidth, central wavelength and pulse energy. Hence the stability of mode synchronization in this type of lasers depends on a feedback loop and its operation based on the electronic control of polarization controllers.
One of the possible configurations of a ring cavity fiber laser is disclosed in U.S. Pat. No. 7,477,665 ('665). The disclosed cavity includes a fiber doped with ions of Er, optical pump energizing the Er fiber through WDM 980/1550 nm multiplexer, two optical polarization controllers, polarization sensitive isolator between the controllers and output coupler. In operation, a small percentage of signal is tapped from the output coupler to be detected by a photodiode and is further converted into frequency dependent signal components including:
A direct current (DC) component (in between consecutive pulses in a pulse generating regime), which corresponds to a continuous mode of operation of the cavity characterized by a narrow spectral line radiation at a central 1550 nm wavelength;
An alternating high frequency component correlated to the mode locking operation with a pulse repetition rate (equal to 1/τ, where τ is the time taken for the light to make one round trip of ring resonator) in a 20 to 100 Mhz range with a spectral line width of about 20 nm and pulse duration of about 200 fs; and
A relaxation low frequency component related to the stability of the mode-locking status; this component is detected in a 50-100 kHz frequency range and also depends from the DC component.
At a start-up stage of the disclosed laser, the electronic control loop is operative to analyze the presence of the above discussed three components and alter the polarization state of at least one polarization controller such that the DC component is decreased to the lowest value, while the other two components tend to increase. Once the relaxation component becomes stable and since the relaxation and alternating components are correlated, an optimal mode locking operational stage of the laser is achieved with uniformly shaped femtosecond pulses.
In operation, the bandwidth, including a central frequency, of laser emitted pulses tends to shift. If this undesired phenomenon is not controlled, the ring laser eventually stops operating in a mode-locked regime. Yet the '665 patent does not anticipate such a possibility and therefore the disclosed in this reference control means are not configured to deal with this problem and thus cannot adjust the pulse bandwidth. Since the duration of a pulse is inversely proportional to a spectral width of the pulse, i.e., the shorter the pulse the broader the spectral width and conversely, the '665 patent clearly does not disclose a structure capable of controlling the duration of pulses either.
Another mode synchronization method for a mode locked ring laser is disclosed in KR20120058275 teaching an automatic feedback loop-based control scheme which is operative to process a sub-region of the entire pulse spectrum. The sub-region to be processed is located outside a pass-band of photodiode integrated into the feedback loop. The disclosed system is not operative to adjust the pulse bandwidth and thus pulse duration in a mode locked regime.
A need therefore exists for a mode-locked fiber ring oscillator based on NPR which has a control feedback loop which is configured to monitor and control a bandwidth of pulse so as to maintain the desired bandwidth essential for the stable mode locked operation of the oscillator.